There is known an interchange judging device which detects the difference between power demand and purchased power which cannot be predicted in the stage of creating a plan. When the quality of power cannot be maintained, the interchange judging device determines, in real time, the electric energy (i.e., power) quantity to be interchanged and another power system supply-demand controller requiring the power with minimum cost.
Each customer makes a contract about power demand having a predetermined value, and whether electric energy exceeding the contracted electric energy is used is judged by the interchange judging device.
There is a difference between the contracted power demand and actually required power demand, and thus the contract should be made considering the power demand not to be actually consumed. Therefore, there is a problem that this interchange judging device unnecessarily creates a plan for generating a great quantity of power. Excessive power generation requires a large-scale power storage to store the excessive power.
Further, when power is supplied from unstable power generating source such as solar power generating unit or wind-power power generating unit, there may be a case where a predetermined contracted electric energy cannot be supplied depending on the weather or weather conditions. There is a problem that the interchange judging device functioning in real time easily generates a request for additional power with strong possibility. Excessive supply of additional power requires a large-scale power storage store the excessive power.
Further, the following is known as another prior art. In this prior art, the target output value of each power generating unit is generated from the total output of power generation based on a load allocation calculation method. In this case, an expected value showing the predictive output value of power generation and a dispersion value representing the reliability of the expected value are used. When the reliability of the predictive value is high, the target output of power generation is calculated, by economic load allocation, to be highly economical requiring minimum fuel cost. On the other hand, when the reliability of the predictive value is low, reserve power generation of a distributed type power source is secured to calculate the target output of power generation with high flexibility.
In this prior art, the instability of solar power generation and wind-power power generation is expressed as reliability, and corresponding load allocation is changed depending on this reliability. In this case, the instability of the demand itself is not considered at all.
Accordingly, similarly to the above prior art, power not to be actually consumed is generated, which leads the possibility of generating excessive power as a result. Further, there is a problem that a large-scale power storage must be built and kept in the system to store the excessive power generated in this case.